From Remedy to Risk: A Case of Metabolic Acidosis Triggered by Sodium Thiosulfate

Introduction: Calciphylaxis is a rare but debilitating vascular condition characterized by small-vessel calcification and skin necrosis that is commonly associated with advanced CKD and end-stage renal disease (ESRD). Sodium thiosulfate (STS) has been used to treat calciphylaxis, with some patients showing lesion and pain improvement. STS has rarely been associated with high anion gap metabolic acidosis (HAGMA), as illustrated in a unique case below. Case Description: A 51-year-old male with a history of diabetes, hypertension, and ESRD on dialysis presented with one day history of diarrhea. Vital signs showed a blood pressure of 73/63 and a heart rate of 84. Labs showed a bicarbonate of 16, anion gap of 21, white count of 29.90, and lactic acid of 9.33. CT of the abdomen revealed distal small bowel thickening, and C. difficile testing was positive. He was treated with IV norepinephrine, oral vancomycin, and IV metronidazole and was admitted to the ICU for septic shock secondary to C. difficile colitis. Within 24 hours, his hemodynamic status improved and labwork returned to normal limits, and vasopressors were discontinued. During the hospitalization, he was found to have a penile lesion concerning for calciphylaxis, prompting initiation of STS. The day after his first dose of STS, he developed a new HAGMA of unexplained etiology. His bicarbonate was 11, anion gap 22 with normal lactic acid and glucose levels. Arterial blood gas showed a primary metabolic acidosis with anion gap. The patient’s HAGMA improved slowly with initiation of a bicarbonate drip and continuation of dialysis, suspicious for STS induced HAGMA. Discussion: While it is generally well-tolerated, STS has rarely been associated with mild to severe HAGMA, especially in patients with impaired renal clearance. Possible mechanisms include accumulation of thiosulfate and its acidic metabolites, impaired sulfate excretion, and mitochondrial dysfunction via generation of hydrogen sulfide. With growing use, it is important to be aware of potential life-threatening acidosis following high doses of STS. Symptoms may include confusion, hypotension, and seizures. Management typically involves supportive care, bicarbonate therapy, and intensified dialysis when indicated. Given the potential severity of this complication, clinicians should closely monitor acid-base status and adjust dosing accordingly.